Tripartite motif or TRIM proteins comprise the largest superfamily of RING-domain E3 ubiquitin ligases. These enzymes function in a wide variety of important cellular processes, particularly in innate antiviral response mechanisms. A defining feature of TRIM proteins is that they are composed of multiple domains, with each domain conferring a specific biochemical functionality to the protein. For these studies, we have a special focus on TRIM5j, which functions in the cell as a restriction factor that inhibits HIV-1 replication. We propose to: (1) Determine the structure of a complete tripartite motif in order to understand the molecular details of how the different constituent domains integrate structurally with each other. We will also determine the molecular principles that govern dimerization and higher-order assembly, which are important elements of TRIM protein function. (2) Define how the different domains of TRIM5j coordinate their biochemical activities, in order to recognize the incoming capsids of HIV-1 and restrict viral replication. We will use a multi-component, integrative approach that combines structural, biochemical, and cell biological techniques. The expected outcome is a comprehensive, molecular level understanding of TRIM5j-mediated inhibition of HIV-1 replication.